I work on the principle that I am a guest in France, therefore I endeavour to follow the rules as best as I can. I put them on for a trip and discard them at the end, so that is 2 sets each year, the cost of a bit of cheese and a few Leffe bottles!
I suspect you will not have any issues unless you transgress the law and have interactions with the police. This year I got pulled over on a motorway en route from Montpellier north after 6 weeks in Spain. As it happens they were customs officers, who wanted to know if we had cigarettes, alcohol, guns! And if we had been to Andorra. That is the sort of scenario when an officious gendarme might decide to look for infractions of the law.
With regard to Canterbury park and ride and others generally not displaying, I remind you of the saying your granny probably used when you were young. “If so and so stuck his head in the fire, would you”?
Davy